A new study suggests that Saturn’s moon Titan has a thick, rigid crust of ice, a finding that confounds scientists whose explanations for the moon’s dynamic surface and atmosphere rely on a flexible crust.

“Already things on Titan were hard to explain. This makes it even worse,” says study coauthor Doug Hemingway, a planetary geophysicist at the University of California, Santa Cruz. “It deepens the mystery of a very strange bod...